Allow null as a synonym for {} when decoding JSON objects.

pkg/analysis_server/tool/spec/codegen_dart_protocol.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library codegen.protocol;
 
 import 'dart:convert';
 
 import 'api.dart';
 import 'codegen_tools.dart';
@@ skipping 520 matching lines @@
    * Emit the method for decoding an object from JSON.
    */
   void emitObjectFromJsonConstructor(String className, TypeObject
       type, ImpliedType impliedType) {
     String humanReadableNameString = literalString(impliedType.humanReadableName
         );
     writeln(
         'factory $className.fromJson(JsonDecoder jsonDecoder, String jsonPath, Object json) {'
         );
     indent(() {
+      writeln('if (json == null) {');
+      indent(() {
+        writeln('json = {};');
+      });
+      writeln('}');
       writeln('if (json is Map) {');
       indent(() {
         List<String> args = <String>[];
         List<String> optionalArgs = <String>[];
         for (TypeObjectField field in type.fields) {
           String fieldNameString = literalString(field.name);
           String fieldAccessor = 'json[$fieldNameString]';
           String jsonPath = 'jsonPath + ${literalString('.${field.name}')}';
           if (field.value != null) {
             String valueString = literalString(field.value);
@@ skipping 272 matching lines @@
   CodegenProtocolVisitor visitor = new CodegenProtocolVisitor(readApi());
   return visitor.collectCode(visitor.visitApi);
 });
 
 /**
  * Translate spec_input.html into protocol_matchers.dart.
  */
 main() {
   target.generate();
 }